Sunak's pledge in a meeting with Brazilian President Luiz Inacio Lula da Silva at 10 Downing Street follows a similar commitment by U.S. President Joe Biden earlier this year, with them joining previous donors Norway and Germany.
"There are so many interests we have in common," Sunak told Lula, mentioning higher trade and the fight against climate change. "I'm delighted to announce we will be investing in your Amazon Fund and I pay tribute to your leadership in this initiative".
It was not immediately clear how much Britain would donate.
The Amazon Fund, first launched in 2009, works to combat deforestation in the Amazon rainforest and spur sustainable development. It was re-activated by Lula this year after his predecessor Jair Bolsonaro froze it in 2019.
British environment secretary Therese Coffey had already told Reuters that Britain was considering joining the initiative.
Last month, the United States government announced it would request $500 million over five years to contribute to the fund and related activities. France and Spain have shown interest in contributing, according to Brazil.
(Reporting by Gabriel Araujo and Pedro Fonseca; Editing by Steven Grattan)
